rhi: Fix offscreen manual test's Vulkan init

Pick-to: 6.5
Change-Id: Iaf7da78fd0c1c1265b05bfd4c7ced5d94ae963fe
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

tests/manual/rhi/offscreen/offscreen.cpp

@@ -137,6 +137,7 @@ int main(int argc, char **argv)
     if (graphicsApi == Vulkan) {
         QLoggingCategory::setFilterRules(QStringLiteral("qt.vulkan=true"));
         inst.setLayers({ "VK_LAYER_KHRONOS_validation" });
+        inst.setExtensions(QRhiVulkanInitParams::preferredInstanceExtensions());
         if (inst.create()) {
             QRhiVulkanInitParams params;
             params.inst = &inst;